Thread checking

[mklotz]

The formula for determining tap drill size while allowing for depth-of-thread is:

TD = MD - 0.013*DOT/P

where:

TD = tap drill size
MD = major diameter of thread
DOT = depth-of-thread expressed as percentage
P = pitch of thread expressed in tpi

The simpler, more commonly used formula,

TD = MD - 1/P

assumes

0.013*DOT = 1

which leads to

DOT = 1/0.013 = 77 %

In all but the most critical applications, you don't need anywhere near that much engagement and it just makes threading much more difficult and leads to tap breakage.

My DRILL program has a chart for recommended DOT on various materials...

Recommended Depth of Thread:

MATERIAL DOT (%)

MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60

but I find that 65% is a good compromise for almost any thread, large or small. Of course, most of my work is model building so ultimate thread strength is almost never an issue. If you're going to build safety-sensitive stuff, seek professional recommendations for DOT.